Homeostasis is keeping the internal environment of the organism as constant as possible. Water is the most important factor in doing so in all living things including plants. This is because it work as a solvent, thermoregulator, and is involved in nearly all metabolic pathways.

Guard cells in the plants prevent water loss from it in cases of soil dryness or high temperature by closing themselves.

If this does not happen, water will keep leaving the plant by the process of transpiration in these harsh conditions and the plant will not have the important medium for the homeostasis process to continue.